On completing this lesson, you will be able to do the following:
■ Create coincident, perpendicular, and parallel constraints.
■ Recognize and use the point glyph when creating constraints.
■ Use the Autoconstrain feature to apply all geometric constraints
automatically.
■ Delete or relax one or all the constraints applied to an object.
File Name: PD_Deck2.dwg
Command: GEOMCONSTRAINT, AUTOCONSTRAIN
Geometric constraints are applied before dimensional constraints to define
and preserve the general shape of the design. For example, geometric
constraints can specify that two endpoints always remain coincident, or that
specified lines always remain perpendicular. AutoCAD 2010 supports 12 types
of geometric constraints.
Let’s apply geometric constraints to a drawing of another simple backyard
deck plan.
To apply coincident constraints
1 Open the drawing file, PD_Deck2.dwg.
Typically, the first constraints that you apply to a drawing are Coincident
constraints. As the name implies, Coincident constraints provide a way
of making sure that the endpoints of objects always remain joined.
However, the definition is broader. For example, a Coincident constraint
accepts the extension of an object as a valid location. In some cases, a
Coincident constraint is similar to a Nearest object snap.
Let’s create a Coincident constraint between the endpoints of the two
lines.
2 On the Geometric panel, click Coincident.
3 Click near the endpoints of the horizontal and the vertical lines as shown.
This action creates a Coincident constraint that looks like a small blue
square at the corner.
A red point glyph is displayed for each selection. These glyphs work like
object snaps, but are limited to Endpoint, Midpoint, and Center. To create
a Coincident constraint at a T-intersection between two lines, use the
Object option.
4 On the Geometric panel, click Coincident again.
5 At the Command prompt, press Enter to specify the Object option.
6 Select the horizontal line as shown, and then select the vertical line near
the endpoint. Make sure that the endpoint glyph is displayed.
You could continue applying Coincident constraints to the other
intersections manually, but there is a faster way to apply them.
7 On the Geometric panel, click Coincident again.
8 At the Command prompt, enter a to specify the Autoconstrain option.
9 Select all the objects in the deck plan and press Enter.
All the Coincident constraints are applied. We will now add the rest of
the geometric constraints.
To apply perpendicular constraints
1 On the Geometric panel, click Perpendicular.
2 Select the horizontal and vertical lines as shown.
The order in which you select the two lines makes a difference in some
cases. For example, imagine two lines in a V-shape with a Coincident
constraint at their intersection. When you apply a Perpendicular
constraint, the first line that you select is the reference line. The other
line will adjust to the first line. This behavior also applies to several other
types of geometric constraints.
3 On the Geometric panel, click Perpendicular again.
4 Select the horizontal and vertical lines as shown.
Notice that the two lines do not need to intersect at their endpoints to
accept the Perpendicular constraint. In some cases, the lines might not
even touch, but can intersect at an extrapolated intersection.
There are several lines in the deck plan that should always be parallel.
We will add those constraints next.
To apply parallel constraints.
1 On the Geometric panel, click Parallel.
2 Select the two horizontal lines as shown.
Parallel constraints do not constrain the lengths and offset distances of
these lines. They only make sure that the lines remain parallel.
3 Click Parallel again.
4 Select the two vertical lines as shown.
5 Go ahead and apply Parallel constraints to each of the four lines of the
stairs. Use the right-most of the four lines as the reference in each case:
1 and 2, 1 and 3, and 1 and 4. You might need to zoom in to select the
lines.
It is not critical to make all the lines parallel to the reference line, but
this method captures the design intent.
To apply fix constraints to the drawing
A Fix constraint locks a specified point on an object to a fixed WCS location
in the drawing. It can also lock a specified object to a limited range of freedom.
Applying Fix constraints typically is used for one of two purposes:
— Permanently locate an object within a drawing so it cannot be moved or
rotated.
— Temporarily hold an object or a point on an object in place while you
manipulate other, partially constrained objects.
You can apply Fix constraints to several objects or points on objects in a
drawing.
1 On the Geometric panel, click Fix.
2 At the Command prompt, enter o to specify the Object option.
3 Select the horizontal line on the deck plan as shown.
This constraint keeps the selected line segment collinear with an
imaginary infinite line, but it does not lock the locations of its endpoints.
Let’s apply a Fix Point constraint to an endpoint of the same line.
4 On the Geometric panel, click Fix.
5 Click the line near the left end.
The second lock icon confirms that the line is now fixed in place. It
cannot move or rotate.
To remove constraints
There are several method you can use to remove constraints.
1 Right-click the Fix constraint that you just added and click Delete.
Alternatively, you can move your cursor over a constraint icon and press
the Delete key. This action removes a single constraint.
2 On the Manage panel, click Delete Constraints.
3 Select one of the lines.
All the constraints applied to that line are removed. Alternatively, if you
are working within a command and one or more constraints prevent you
from making a change, press and release the Ctrl key to relax the
constraints on the selected objects. After completing the command, the
constraints will be reapplied, if possible.
4 On the Manage panel, click Delete Constraints.
5 Select all the lines using a Window or Crossing selection.
This action removes all the constraints from the deck plan.
To constrain selected objects automatically
The Autoconstrain feature adds a complete set of geometric constraints to
selected objects automatically. Let’s try it on the deck plan.
1 On the Geometric panel, click Auto Constrain.
2 Select all the lines using a Window or Crossing selection.
All the objects are geometrically constrained to each other. You can now
make adjustments, deleting the constraints that you do not want to use
and applying others.
Notice from the drawing that Autoconstrain assumed that the bottom
horizontal line should always remain horizontal, and that the top step
should always be parallel to the right edge of the deck. These and other
assumptions are not always going to be correct for your designs. You
should always plan to make some adjustments after using Autoconstrain.
3 Close the drawing without saving it.
Summary
You have learned how to apply several types of geometric constraints to capture
the intent and requirements of the design. However, geometric constraints
do not fully constrain the size and shape of the objects in a drawing. To fully
constrain a drawing, you need to apply dimensional constraints, which is the
subject of the next lesson.
Review Questions
1 What is the purpose of a coincident constraint?
2 What is the significance of the first object selected when you apply some
types of constraints?
3 What are Fix constraints?
4 What is a fast way to remove all the constraints from a selected object?
5 When is it recommended to use Autoconstrain?
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